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Last October, at a rally in Las Vegas, a friend of Donald Trump's introduced the mogul-turned-candidate with a story about Trump's generosity.

"You won't hear this in the media, but Donald gave $20 million to the St. Jude children's home. Twenty million dollars," said Phil Ruffin, the owner of the Treasure Island casino, which was the site of the rally.

The crowd cheered. Trump mouthed "Thank you" twice and waved.

"He could have used that $20 million for television ads, but he decided to give with children with cancer," Ruffin said. "That's Donald Trump."

Afterward, Trump spoke for more than an hour. He talked up his poll numbers ("I've been leading since I came out."). He rehashed a recent GOP primary debate. He lambasted his Republican rivals, Hillary Clinton, China and the news media ("They're such dirty, rotten liars, it's disgusting," he said, criticizing TV news channels for not showing the size of his crowds).

He did not mention anything about the $20 million gift.

Later that day, however, Trump retweeted a message from a fan, criticizing the mainstream media for not broadcasting Ruffin's story about the gift.

The implication, from Trump, was that the gift story was important (and, therefore, true).

If Ruffin's story were true, then Trump's gift to St. Jude would appear to be, by far, the largest charitable donation of Trump's life.

But when The Washington Post looked for evidence to back up Ruffin's story it could find none.

At St. Jude's Children's Research Hospital, a world-famous cancer center in Memphis, spokeswoman Kelly Schulz said they had never announced a large gift of this kind from Trump. St. Jude's does not release detailed information about donors, so The Post could not determine what - if anything - Trump has donated to St. Jude's from his own pocket.

A spokeswoman for Ruffin, Michelle Knoll, said she had seen no evidence that would verify Ruffin's claim. She said Ruffin was out of the country now, and could not be reached. She said that Ruffin's staff had heard him repeat the story on other occasions.

Trump's presidential campaign did not respond to requests for comment.

Records show Trump has donated to St. Jude's in the past, but never on the scale of the gift in Ruffin's story.

The Donald J. Trump Foundation, a nonprofit of which Trump is president, has donated $60,000 to the hospital. Its last recorded gift was in 2008, when Trump gave $50,000 - apparently in connection with an episode of "The Celebrity Apprentice." In addition, a list of charitable donations by Trump's companies, provided by the Trump campaign last year, lists $92,603 worth of gifts to St. Jude over five years. In many cases, these seemed to be "in-kind" gifts, like free rounds of golf or gift certificates that Trump properties donated for raffles and fundraisers.

Trump has also declined to release his tax returns, which would make it possible to see what Trump has given personally to this cause - and many others.

It seems possible that what Ruffin was referring to actually involved another Trump.

Eric Trump, the candidate's 32-year-old son, runs a foundation that raises money for St. Jude's through an annual charity golf tournament. It receives some help from his father, who donates the use of golf course facilities, and has given $150,000 out of the Donald J. Trump Foundation. But it is a separate entity: Tax forms show Donald Trump is not on its staff or a member of its board. In a recent interview, Eric Trump told The Washington Post that he believed his father had also given his own cash to the Eric Trump Foundation. But Eric Trump could not recall a specific time when that had happened.

In February 2015 - a few months before Ruffin's comments - Eric Trump's foundation announced that it had acquired naming rights to a surgical and intensive-care center at St. Jude's. To get those rights, Eric Trump's foundation made a pledge to raise a large amount money for St. Jude's over the next 10 years.